The feeding of copy sheets to the supporting surface of the bed is conventionally accomplished by grippers which seize the sheet and entrain it across the bed into printing position. The grippers must then be retracted relatively to the bed so as not to interfere with the lowering of the screen holder into its working position. For this purpose, either the sheet grippers or the bed must be vertically movable.
Aside from the need for providing a rather complex gripper drive or a separate drive for the printing bed, the relative vertical motion between the supporting surface of the bed and the grippers engaging a sheet edge tends to wrinkle or crease the sheet and to mar the appearance of its imprinted face.